Method of and composition for producing electroless black nickel coatings



.starting layer of some tion of Connecticut No Drawing. Filed June f4, 1061, Ser. No. 116,958

8 Claims. (Cl. 1486.14)

The present invention relates to the formation of adherent black coatings on metallic nickel, including nickel alloys containing 50% or more of nickel; and more particularly it relates to solutions for and methods of producing such a coating on nickel through reaction therewith without the use of electric current. Objects fabricated of sheet or cast nickel may be treated in accordance with this invention, so long as these contain at least the usual trace amounts of impurities such as sulfur, arsenic, bismuth, etc., which promote activation for blackening purposes of the nickel in the treatment solution. The great preponderance of items, however, for which the invention finds its principal application are those which include metallic nickel in the form of an electrodeposited plate on the surface of a basis metal. Also included in this category are non-metallic objects which have been electroplated with nickel after initial deposition of a thin metal by any one of several known chemical methods. The terms nickel or metallic nickel as used herein refer therefore to metallic nickel or nickel alloys containing at least 50% nickel in any of these forms. It does not, however, include nickel in nonmetallic form, such as electroless nickel plates or deposits which have an amorphous structure and do not readily lend themselves to blackening treatment by the solutions and methods herein taught.

Black nickel finishes are frequently desired in industry for a variety of reasons, such as improved appearance, better corrosion resistance, or as a preparatory step to painting, waxing, lacquering, etc. Among the many possible fields of application, there may be mentioned jewelry, electronic components, hardware items, precision tools, cameras, projectors, microscopes and similar optical equipment.

Processes for producing black nickel coatings have long been used in industry, but these have required the use of electrolysis. Such processes present certain difiicul-ties inasmuch as stringent conditions must be maintained in order to produce black deposits of uniform quality. Furthermore, electrolytic methods of depositing uniform black nickel coatings are diflicult to control on very small parts immersed in the solution in bulk because of unavoidable conditions of random, non-uniform current density distributions on the parts being treated. Another limiting factor of the electrolytic method is that the black deposit of-ten lacks satisfactory adhesion to the basis metal and is readily removed by mild mechanical or chemical action.

It is accordingly one of the principal objects of this invention to provide a protective and decorative black coating on nickel as hereinabove defined without the use of electric current, by simply immersing the parts to be blackened in, or otherwise contacting them with, a suitable solution. Thus, since no electric current is required, the parts to be blackened can be immersed in the solution by any method which permits easy access to the solution, such as the use of conveyor systems for continuously passing the parts through the solution on racks or in foraminous baskets, or by tumbling in a barrel or tank of the solution. This black finish can be imparted to nickel as above defined, or nickel plated parts, including non-metallic base parts which have been metalized.

3,097,117 Patented July 9, 1963 ice Another object of this invention is to provide an aqueous blackening solution which is simple to operate and which can be readily and safely handled in production operations by untrained personnel.

A distinct advantage of the invention is that black nickel coatings can be successfully applied to old, passive nickel deposits with or without preparatory cleaning, which permits processing of nickel plated objects that have been stored for extended periods of time, as fre quently occurs in industry. It is an inherent property of the blackening formulations disclosed herein that they have substantial capacity to re-activate nickel surfaces by removing passive nickel layers during the initial period of immersion in the solution, thus overcoming difliculties frequently encountered heretofore due to superficial nickel oxide films.

In accordance with the present invention, nickel objects, whether in sheet or other fabricated form, or having an electroplated deposit over a basis metal, may be given an adherent black coating by simply immersing them without the use of electric current in an aqueous solution containing an oxidizing agent belonging to the family of aromatic nitro derivatives and an inorganic salt of thiccyanic acid, together with a sufiicient amount of a strong inorganic acid material to lower the pH of the solution to 2.0 or less. While some blackening will occur at room temperature in such solution, for all practical purposes the solution should be maintained at a minimum temperature of about F. The process is operative at any temperature above this up to the boiling point of the solution. In all cases the system will be suitable for blackening of nickel only so long as the pH is lowered to 2.0 or less, which may be accomplished by the addition of a suitable mineral acid. Various strong rnineral acids are adequate, such as nitric or phosphoric, but sulfuric acid and its mono salts are preferred. The hydrogen ion concen tration in solution should be at least 0.01 equivalent per liter, which will produce a pH of approximately 2.0, while higher concentrations are compatible with good functioning of the blackening solution up to around 1.0 equivalent per liter. Acid concentrations greater than 1.0 N give undesirable results and are outside the scope of the invention.

The aromatic nitro derivatives found useful for this invention include the water soluble sulfo, amino, hydroxy and carboxy substituted nitrobenzenes and nitronaph-thalenes. Preferred compounds within this group include sodium m-nitrobenzene sulfonate and sodium m-nitrobenzoate or the corresponding acids.

The thiocyanates found effective are the ammonium, sodium and potassium salts of the acid.

The following concentration ranges are successfully employed in applying black nickel coatings in accordance with this invention:

7 Mole per liter Aromatic nitro derivative 0.05 to 1.0 Inorganic thiocyanate 0.01 to 0.5

For purposes of further illustration of the invention, a preferred specific aqueous solution has the following composition Grams per liter Sodium m-nitrobenzene sulfonate 60 Ammonium thiocyanate 5 Sulfuric acid (66 Baum) 2 Such a solution, when used at temperatures from 100 F. to boiling, will produce attractive and adherent black coatings on metallic nickel objects by immersing the object in the solution for a short but sufiicient period of time, usually of the order of 15 seconds to 5 minutes depending on the depth of color desired. Agitation of the solution or the objects will shorten the treatment time and is desirable but not essential. e

A solution preparedin accordance with the foregoing specification, maintained at a temperature of 140 to 160 F., was employed in conducting each of the following tests:

Example 1 A brass panel plated with a 0.0002" deposit of bright nickel was immersed in the above solution. It was observed that the panel first turned a light brown which then deepened int-o an intense black after 1 minute of immersion time. Longer exposure to the solution resulted in further deepening of the black color which reached its highest intensity after 3 minutes, beyond which no noticeable change could be observed. The smoothness of the original nickel plate remained unaltered during the blackening operation which produced a black coating with high gloss.

Example 2 A steel panel plated with a 0.0003" deposit of bright nickel was immersed in successive steps in the above formulation at 150 F While observing the effect at different immersion times ranging from 15 seconds to 3 minutes. It was found that after the initial 15 seconds, the coating turned an iridescent blue which, on longer exposure to the solution, changed to an intense black. The black was W611 adhered and after drying formed a suitable basis for painting, waxing, or other supplemental metal finishing operations.

Example 3 A specimen of zinc die casting was plated with a copper and then with a nickel deposit, and submitted to the above solution for about 2 minutes at approximately 150 F. The treatment resulted in a uniform, pleasing black color in the nickel deposit.

Example 4 A copper basis metal item coated with bright nickel was immersed in the above formulation for 2 minutes, which gave the object a deep black color. After light bulfing, the nickel took on a glossy, gray silvery appearance.

Example 5 A panel of Inconel alloy was adequately cleaned and immersed in the above formulation at 160 F. for 4 to minutes. It was noted that blackening proceeded more slowly than with electrodep-osi-ted nickel coatings. How ever, after contacting the specimen with the solution for the above period of time, a satisfactory black coating developed on the surface of the panel.

In order to minimize the handling and shipping risks, it is frequently desired to provide a dry composition which can simply be added to water at such time as it is desired to treat the nickel or nickel plated objects to produce the black nickel coating thereon. Such a composition within the scope of tthis invention is readily provided as follows:

I Parts by weight Sodium m-nitr-obenzoate 12 Ammonium thiocyanate a 1 Sodium bisulfate 6 This composition, when added to Water in amount sufiicient to bring the resulting solution to a pH of 2.0 or less, operating at a solution temperature of around 140 to 160 F., produces very satisfactory results. Tests correspending to those enumerated in Examples 1 through 5 above, substituting the last-mentioned formulation for that initially described in the foregoing examples, produces equivalent results in all cases.

Where the work to be blackened has just come out of the nickel plating bath, it should be carefully rinsed in cold water, whereupon it may then be immersed directly in the black nickel solution for the appropriate length of time. The dissolution of the nickel in solutions of the invention occurs at the rate of 0.00001" per minute at 140 F. Therefore, any Work which has a nickel plate of minimum thickness of at least 00002-00003 can be safely blackened. Following immersion in the blackening bath, the work is again rinsed in cold Water and then allowed to air dry. The intensity of the black color is controlled by the length of the immersion time, increasing with longer periods of immersion. As already mentioned, the black coating may be subjected to further supplemental finishing, such as Waxing, lacquering, oiling, color b-ufiing, etc., to produce a pleasing satin appearance on the treated surface.

The solutions of the invention have exceptional capacity for accepting nickel goods whose surfaces have become oxidized through extended periods of storage. In such cases, the periods of immersion Will generally be somewhat longer than those recommended hereinabove. If desired, the oxidized surfaces may be given a preliminary treatment, as by first subjecting them to soaking action in any of a number of conventional alkaline cleaning solutions, then rinsing in water, followed by an acid dip and a further rinsing cycle.

Good rinsing of the parts after blackening is important in producing a uniform finish.

The invention has been described in connection with certain preferred embodiments but it is to be understood that these are illustrative rather than restrictive of the exact scope of the invention which is set out in the appended claims.

What is claimed is:

1. An aqueous solution for producing an adherent, black coating on nickel by reaction therewith, consisting essentially, in addition to water, of from 0.05 to 1.0 mole per liter of a water soluble compound selected from the group consisting of the sulfo, amino, hydroxy and carboxy substituted nitrobenzenes and nitronaphthalenes, from 0.01 to 0.5 mole per liter of a member selected from the group consisting of the ammonium and alkali metal salts of thiocyanic acid, together with a sufiicient amount of a member selected from the group consisting of the mineral acids and the acid salts thereof to produce a hydrogen ion concentration in the solution of from 0.01 to 1.0 equivalent per liter.

2. An aqueous solution for producing an adherent, black coating :on nickel as defined in claim 1, wherein the substituted nitrobenzene compound is sodium m-nitrobenzene sulfonate, the thio-cyanic acid salt is ammonium thiocyanate and the mineral acid is sulfuric.

3. An aqueous solution as defined in claim 2, wherein there are present in one liter of the solution approximately 60 grams of m-nitrobenzene sulfonate, 5 grams of ammonium thiocyanate and 2 grams of sulfuric acid (66 Baum).

4. An aqueous solution for producing an adherent, black coating on nickel by reaction therewith consisting essentially, in addition to Water, of. approximately 60 grams of sodium m nitro benzoate, approximately 5 grams of ammonium thiocyanate and approximately 30 grams of sodium bisulfate, per liter of solution.

5. A dry composition for use in preparing an aqueous solution for producing an adherent, black coating on nickel by reaction therewith, said composition consisting essentially of twelve parts of sodium m-nitro benzoate, one part of ammonium thiocyanate and six parts of sodium bisulfate, all of said parts being by weight.

6. The process of producing an adherent, black coating on nickel, which comprises immersing the nickel in an aqueous solution of the composition defined in claim 1, for a period of from 15 seconds to 10 minutes, While maintaining the temperature of the solution at from F. to boiling.

7. The process of producing an adherent, black coating on nickel, which comprises immersing the nickel in an aqueous solution of the composition defined in claim 3, for :a period of from 15 seconds to 10 minutes while maintaining the temperature of the solution at from 100 F. to boiling.

8. The process of producing an adherent, black coating on nickel, which comprises immersing the nickel in 211211 aqueous solution of the composition defined in claim 4,

References Cited in the file of this patent UNITED STATES PATENTS Singler May 25, 1954 Mitchell Nov. 15, 1960 

1. AN AQUEOUS SOLUTION FOR PRODUCING AN ADHERENT, BLACK COAATING ON NICKLE BY REACTION THEREWITH, CONSISTING ESSENTIALLY, IN ADDITION TO WATER, OF FROM 0.05 TO 1.0 MOLE PER LITER OF A WATER SOLUBLE COMPOUND SELECTED FROM THE GROUP CONSISTING OF THE SULFO, AAMINO, HYDROXY AND CARBOXY SUBSTITUTED NITROBENZENES AND NITRONAPHTHALENES, FROM 0.01 TO 0.5 MOLE PER LITER OF A MEMBER SELECTED FROM THE GROUP CONSISTING OF THE AMMONIUM AND ALKALI METAL SALTS OF THIOCYANIC ACID, TOGETHER WITH A SUFFICIENT AMOUNT OF A MEMBER SELECTED FROM THE GROUP CONSISTING OF THE MINERAL ACIDS AND THE ACIDS SALTS THEREOF TO PRODUCE A HYDROGEN ION CONCENTRATION IN THE SOLUTION OF FROM 0.01 TO 1.0 EQUIVALENT PER LITER. 